The present invention relates generally to imaging systems. More particularly, the present invention relates to ultrasound imaging systems.
There are a number of disadvantages associated with various imaging systems that are currently in use, particularly when used for medical applications. For example, a number of imaging techniques, such as x-ray imaging, mammography, and computed tomographic (CT) scans, use ionizing radiation that presents a risk of cell mutation when used medically. Also, CT scans and magnetic resonance imaging (MRI) techniques both involve procedures that are relatively expensive, a factor that by itself acts to some degree to limit their use. A significant disadvantage of methods such as mammography is that they rely on two-dimensional images that may disguise three-dimensional structure information that can be critical for diagnosis.
As an alternative to these imaging technologies, the medical community has looked to ultrasound for providing a safe, low-cost, high-resolution imaging tool. There are, however, significant limitations to conventional ultrasound, which may be used in A or B scanning modes. Such modes are distinguished by the fact that an A scan is purely one dimensional while a B scan produces a two-dimensional image. As a result, imaging applications tend to use ultrasonic B scanning. In such conventional ultrasound analysis, a small array of elements is moved by hand in contact with tissue under study. The array sends out waves that reflect from tissues back to the same array. This arrangement results in two major drawbacks. First, ultrasonic B scans do not provide information on the properties of the materials themselves; rather, they provide information only on the reflectivity of the boundaries between different types of materials. Second, the array is incapable of capturing radiation except that reflected back to the hand-held sensing array. Considerable information exists, however, in the transmitted waves, but this information is neither captured nor used diagnostically in conventional ultrasonic B scans.
An additional limitation to traditional ultrasound techniques is that when an unknown object is examined, it is difficult to determine success criteria for the image construction. Thus, it would be useful to be able to benchmark the image construction process in order to determine when sufficient accuracy or precision has been obtained. Moreover, in the past, it has been difficult to correlate positions of features in an image with the position of the patient. It therefore is desirable to develop methods of judging the accuracy of an ultrasound image against objective criteria and adjusting the image to correlate to those criteria.
Another useful application for ultrasound imaging is analyzing changes in a tissue, for example, by creating multiple ultrasound representations of the tissue, perhaps over the course of several days, weeks, months or years. Such analysis is most beneficial, however, if it can be undertaken from a consistent frame of reference, such that the size and orientation of the tissue and any features therein are consistently depicted in each representation. Thus, in creating an ultrasound representation, it would be desirable to develop a method of comparing multiple ultrasound scans from a consistent frame of reference.
There is thus a need for an apparatus and method that provides improved imaging, particularly as applied to medical applications.